NASA AIS performance - an aerial problem ??

[CPD]

I have recently fitted the NASA AIS Radar - lets not get into the radar/ais discussion again tho. My question relates to aerials and what type/location people are using to get the best result. I haave a whip aerial on the pushpit which gave very duissapointing results. An emergency aerial strapped to one of the shrouds seems better but not brilliant. What have you found to work
best ??

 

[alan]

I have a 1m (I think) Glomex antenna on the pushpit. I use an Easy AIS.
Sitting in the marina surrounded by other masts e.t.c., I am able to pick up ships at 18M where I have "line of site". Typically the ships are going from/to Naples from the north Italian area, and passing my marina at about 5 to 7 miles off the coast.
I have yet to do a range test when I am sea with 360 degs unrestricted visibility.

Alan.

 

[alldownwind]

Yes, don't forget that VHF basically works within line of sight. A whip on the pushpit is really low down, and if you have anything else immediately around you the reception is going to be even more restricted.
Higher the better.........

 

[Endeavourquay]

I recently fitted my Nasa AIS Radar and used a 1m Glomax on the pushpit rail of my little Contessa, It is picking up boats up to 20 miles away whilst in the Portsmouth Harbour and still does well in the Marina. The Longer range test will be at the weekend. /forums/images/graemlins/grin.gif

 

[DipperToo]

Just fitted an EasyAIS with a small 'stubby' pushpit aerial. Picks up targets 19 miles away when I am in the marina (Shotley) surrounded by many other boats.

 

[hubert]

I have the nasa ais radar. With a 1 metre whip about 3 metres above sea level I get well in excess of 20nM.
It is important you set the thehold correctly. Signals below the threshold will not be recorded. I set mine just one stop above the quiescent level. Setting it higher clears more distant targets from the screen.

 

[Salty John]

Range is line of sight and determined by a combination of transmitter and receiving antenna heights.
Because AIS is transmitted from large vessels with antennas located high above sea level it is less important to have your antenna high up. If your antenna is 10' above sea level and the transmitter is 100' above sea level the theoretical range is 16 nm.
If you aren't getting anywhere near this range you probably have an antenna system fault - typically a bad connection or broken coax, or possibly a duff antenna.

 

[AliM]

With a 1 m whip aerial on the pushpit and NASA AIS, I can see ships in the Thames, when I am in the Crouch (say ~10 miles away). I know that there are no hills in Essex, but that is a bit better than line-of-sight!

 

[fireball]

My engine - now returned to NASA for examination - would only pick up 5 miles (see a recent post of mine) ... I suggest you eliminate the engine being at fault by either testing a known good unit on your setup or your engine on a known good setup ...

Can you keep us informed of your results as we may be able to use this as evidence of a quality issue if necessary (it probably won't come to it, but might as well be prepared).

When did you buy your engine?

 

[captainslarty]

Using the AIT250 Mode B transponder, the supplied antenna is a rubber duck type unit.. mounted on the pushpit.. works perfectly.

 

[CPD]

mmmm interesting. I bought mine about 2 months ago. On a recent jaunt round the coast, there were a number of large vessels within 6 miles (checked on conventional radar) that the ais wasnt seeing. Okay, some vessels may have forgotten to turn the ais on, but there too many of them I think for that to be the case.

Can someone explain exactly how to set the threshold ?. Im sorry, but I have left the instructions on the boat. I know from a friend that the threshold is rather like squelch on VHF which makes sense, but you can hear squelch on vhf.

I may be able to get hold of a similar unit to check if that is the problem, and will let you know.

Thanks for the help and comments chaps.

 

[fireball]

There is no manual squelch on the NASA unit - any changes you make to the software is reset on powerdown anyway ... and the unit supposedly starts off at it's most sensitive....

It sounds like your unit has the same problem as mine - but best to check the antenna just incase ... you could always plug the antenna into the VHF and do a radio check ....

 

[hubert]

The original post concerned a nasa ais radar which has an adjustable threshold setting. ( As one forumite accurately likened to a squelch control.) I believe in the ais engine this level is fixed at one microvolt.

 

[Salty John]

To plug the AIS antenna into a VHF radio you need a PL259 plug on the antenna cable.
An SO239 to BNC male adaptor, available at radio shops, is a worthwhile item because it allows you to use your AIS antenna as an emergency radio antenna.
Both antenna cables (AIS and radio) will have a PL259 plug and the AIS engine would have the adaptor plugged into it's BNC inlet allowing it to take the PL259.
Or make up a short lead with an SO239 socket at one end and a BNC at the other to achieve the same thing.

 

[fireball]

The NASA AIS Engine does indeed have an adjustable threshold setting - you can manually set this, although it does reset when you remove power to the unit ... it resets to the most sensitive and progressively turns down supposedly to eliminate errors.
There is no physical squelch control (not that NASA Marine would admit too anyway) ....

 

[fireball]

Ah .. yes ... I just assumed he'd have the "normal VHF plug" going into an adapter .. as that is what I and many others do ... but just as well to remind us!

 

[syfuga]

We use a whip aerial mounted on a solar panel 'gantry' with an AIS engine. My experience is that its sensitivity varies: sometimes we pick up ships 60nm away, and sometimes appear not to pick up those much closer. We do share the aerial with SSB DSC using a splitter (never proven to work as it happens).

 

[martinriches]

I had a similar problem with my NASA Radar.After spending ages remaking the connections I assumed it must be that the 1m aerial was to short and got a 2m one. Still no difference so I sent the radar back to NASA. It was returned saying they had repaired a fault. I now get an excellent reception, the tip of my aerial is about 3.5m from water.
p.s. They sent me a bill for postage which I ignored and never heard any more.

Martin